Neutrino Bomb' Idea Expands Debate on Human Extinction

By MALCOLM W. BROWNE

Published: January 23, 1996

ALTHOUGH paleontologists agree that the history of life on earth has been punctuated by five great extinction events, the causes of these catastrophes, including the one that killed off the dinosaurs, are subjects of endless controversy. A new hypothesis based on cosmic "neutrino bombs" promises to complicate the debate still more.

If the hypothesis is correct, the human race itself has one more hazard to think about, a danger against which no protection seems possible. It may be that once every 100 million years or so, a flood of all but undetectable subnuclear particles surges through the earth, causing an epidemic of fatal cancers and genetic mutations with dire results for many species.

The scientist who conceived the idea notes that the human race has existed for less than one-twentieth of this span, and the danger, if it exists, is probably remote.

In a paper accepted for publication by the journal Physical Review Letters, Dr. Juan I. Collar, a Spanish astrophysicist at CERN, the European Laboratory for Particle Physics near Geneva, suggests that high-energy cosmic neutrinos spawned by collapsing stars could pose a much greater risk to life on earth than has been supposed.

Neutrinos are tiny particles that have no electric charge and little or no mass. They interact with atoms of matter so rarely that the average neutrino can pass through the Earth (or even the entire universe) without hitting anything.

Part of the neutrino flow reaching the Earth comes from the thermonuclear reactions that fuel the Sun, but there are too few solar neutrinos with high enough energy to cause appreciable biological damage, Dr. Collar said.

But the torrents of neutrinos produced by the quick collapse of massive stars are a more serious matter, he speculates. In such a star's final stage of collapse when it has used up its nuclear fuel, the star's ordinary atoms are crushed by gravity into a kind of super-dense neutron soup, and most of the "binding" energy that had held together the original atomic nuclei is released in the form of neutrino particles.

The collision of any one of these neutrinos with an atom anywhere in the universe is highly unlikely. But because stellar collapses produce such astronomical numbers of high-energy neutrinos, the chances that some would hit other atoms are greatly increased.

Neutrino detectors built at laboratories in various parts of the world usually consist of enormous tanks of water, in which the rare impact of a neutrino produces a tiny flash of light.

When a high-energy neutrino hits an atom, it transfers most of its "recoil" energy to the atom, which then becomes a microscopic but potentially deadly projectile. A recoiling atom can rip deeply into biological tissue, releasing its damaging energy very rapidly along its track, destroying cells essential to life, causing mutations of DNA genetic material, and initiating cancers.

Taking into account calculations by Dr. John Bahcall of the Institute for Advanced Studies in Princeton, N. J., Dr. Collar suggests that about once every 100 million years, a "silent" stellar collapse -- one that does not produce a visible supernova explosion -- may occur close enough to Earth to have catastrophic effects.

By making some assumptions based on known biological effects of different types of radiation, Dr. Collar estimates that besides killing many animals outright, the neutrinos from a close "silent" stellar collapse would produce 12 cancer sites per kilogram, or 2.2 pounds, of body weight. This, he said, would be "an insult that would be severe enough to kill a vast percentage of large animals with a frequency comparable to that of most major extinctions."

The effects of a spectacular nearby supernova would have even more devastating effects, of which neutrino damage would be only one. Fortunately, astrophysicists estimate that the explosion of a relatively nearby supernova is so rare that the odds are that there would have been at most one since life on Earth began some three billion years ago.

Dr. Collar acknowledges that his hypothesis is "purely speculative," especially since biological effects of neutrino impacts have never been identified. "This idea is really an outgrowth of a mass extinction hypothesis developed by John Ellis, a theorist at CERN, and David Schramm of the University of Chicago," he said.

Dr. Ellis and Dr. Schramm made calculations suggesting that a supernova occurring anywhere within about 33 light-years of the Earth would produce a blast of cosmic rays that would destroy the Earth's protective ozone layer and expose its creatures to deadly solar ultraviolet radiation.

"I can't pass judgment on Dr. Collar's hypothesis," Dr. Ellis said in an interview, "but as far as I can tell, he's estimated the figures correctly. We know that neutrinos are produced by stellar collapses, because neutrino bursts were detected in the United States and Japan in 1987 at the same time a relatively nearby supernova flared up. But, of course, we can't be sure of possible biological effects."

Paleontologists who have studied mass extinctions revealed by the fossil record seemed unimpressed with Dr. Collar's "neutrino bomb" hypothesis.